1. Field of the Invention
The present invention relates to a method of manufacturing a printed wiring board, and more particularly, it relates to a method of manufacturing a double-side copper-clad laminated printed wiring board, which is subjected to high density wiring with a narrow through-hole pitch.
2. Description of the Background Art
In general, a printed wiring board for high density wiring is applied to an office automation apparatus such as a personal computer or a facsimile, or a domestic apparatus such as a video camera or a CD player. In order to improve densification of wiring, a multilayer wiring board has already been generally employed. FIG. 1 illustrates a typical double-side copper-clad laminated printed wiring board 1, which is provided with wiring only on front and rear outer surfaces and having no internal wiring layer. Wires 2a and 2b provided on the front and rear surfaces of this printed wiring board 1 are connected with each other through a through-hole 3.
A method of manufacturing such a printed wiring board of the double-side copper-clad laminated type is now described with reference to a base material prepared from glass fabric base epoxy resin.
FIG. 2 illustrates typical sectional structure of a laminate which is formed by a base material of glass fabric base epoxy resin and copper foils adhered to front and rear surfaces thereof. In this laminate, copper foils 6 are provided on front and rear surfaces of the base material, which is formed by an epoxy resin member 4 and a plurality of glass fabric layers 5 arranged therein. The glass fabric layers 5 are adapted to reinforce the base material, since the glass fabric material has high tensile strength and is excellent in voltage resistance and dielectric properties. Such a copper-clad laminate of glass fabric base epoxy resin is widely applied to a printed wiring board for a general office automation system, a digital audio set and the like.
FIG. 3A is a sectional view showing the double-side copper-clad laminate of glass fabric base epoxy resin, which is provided with pierced holes 7 by perforation.
As shown in FIG. 3B, copper through-hole plating layers 8a are formed on inner peripheral walls of the pierced holes 7, and copper plating layers 8b are formed on the surfaces of the copper foils 6 provided on the front and rear surfaces of the base material. The copper through-hole plating layers 8a and the copper plating layers 8b are generally formed in the following manner: First, the surfaces of the laminate are rinsed or dipped in acid or alkaline chemicals to be cleaned, in the state shown in FIG. 3A. Then, the laminate is subjected to electroless copper plating, rinsing and acid treatment, and further subjected to electrolytic plating, to be provided with the copper through-hole plating layers 8a and the copper plating layers 8b.
After the copper plating layers 8a and 8b are thus formed, the copper plating layers 8b and the copper foils 6 provided on the front and rear surfaces of the base material are etched in prescribed patterns, to attain the state shown in FIG. 3C. The wiring patterns formed on the front and rear surfaces of the base material are connected with each other by through-holes 9, which are defined by the copper through-hole plating layers 8a. The copper plating layers 8b and the copper foils 6 are subjected to pattern etching by a tenting method employing dry photoresist films, for example. The patterns are formed by the tenting method as follows: First, the surfaces of the copper plating layers 8b are subjected to pretreatment of levelling by mechanical polishing with a brush or a buff, soft etching with chemicals or the like. Then, the copper plating surfaces are laminated with photosensitive etching resist films, which in turn are exposed to ultraviolet light through photomasks and thereafter selectively removed by a developing solution. Thereafter, the copper plating layers 8b and the copper foils 6 are selectively removed by an etching solution. Finally the etching resist films are removed by a release solution, to attain the state shown in FIG. 3C.
When a printed wiring board is manufactured by the aforementioned method to be provided with high density wiring patterns with a narrow through-hole pitch, the following problems are caused:
In an initial stage, insulation resistance between through-holes is problematically low. In a long-range view, on the other hand, a problem of copper migration, i.e., movement of copper ions, is caused between the through-holes in response to a voltage load applied across the same, to easily result in current leakage.
Such phenomenons are caused by clearances defined between the glass fabric layers 5 and the epoxy resin member 4 on the inner peripheral wall surfaces of the pierced holes 7 after perforation of the copper-clad laminate.
FIG. 4 typically shows a part of a base material, which is held between two through-holes 9. Referring to FIG. 4, clearances 10 are defined between glass fabric layers 5 and an epoxy resin member 4 in the vicinity of inner peripheral walls of the through-holes 9 of a printed wiring board 1 manufactured by the aforementioned method, and such clearances 10 are filled up with parts of through-hole plating layers 8a. As hereinabove described, the copper plating layers 8a are generally pretreated with acid or alkaline chemicals, which fill up the clearances 10. Since the clearances 10 are extremely fine, the chemicals or plating solutions deeply fill up the clearances 10 through capillarity. It is difficult to completely remove the chemicals or plating solutions thus filling up the clearances 10, even if the printed wiring board 1 is cleaned by rinsing or another method. Thus, insulation resistance between through-holes is reduced in an initial state by the chemicals or plating solutions filling up the clearances 10.
When copper through-hole plating is performed without removing the chemicals or plating solutions filling up the clearances 10, small amounts of ionic substances are charged in the clearances 10, which are sealed by copper plating. Therefore, if voltage is applied across the two through-holes 9, copper migration substances 11 are triggered by the charged ionic substances to gradually extend from the plus side toward the minus side. Consequently, the two through-holes 9 are finally shorted to cause a malfunction in the circuit.
The problem of deterioration in insulation resistance of a printed wiring board caused by copper migration has been reported by, for example, J. P. Mitchells et al. of Bell Telephone Laboratories ("Conductive Anodic Filament Growth in Printed Circuit Materials, Printed Circuit World Convention II, '81", for example). This report discloses deterioration of the insulation property following generation of the so-called CAF (conductive anodic filaments) caused by growth of copper ions between through-holes of a printed wiring board of glass fabric base epoxy resin. This literature also reports an acceleration test made under a high temperature, high humidity and high voltage as means for evaluating the insulation property of a printed wiring board.
Such copper migration is a problematic phenomenon which may occur between copper electrodes provided on a substrate surface as an electronic circuit of a printed wiring board is highly densified and the distance between electrode terminals is extremely reduced. FIG. 5 shows the mechanism of copper migration occurring on a substrate surface. The above literature explains this mechanism as follows:
Consider that a water droplet 15 is in contact with an anode-side copper electrode 13 and a cathode-side copper electrode 14, which are provided on a substrate surface 12. First, the anode-side copper electrode 13 ionizes upon contact with the water droplet 15, to liberate cations of copper. The cations migrate toward the cathode side due to potential difference, while receiving a negative load from the copper electrode 14 to react as follows: EQU Cu+4H.sub.2 O.fwdarw.Cu(OH).sub.2 +O.sub.2 .uparw.+3H.sub.2 .uparw.
In this case, Cu(OH).sub.2 and O.sub.2 gas are generated in the vicinity of the anode-side copper electrode 13, while H.sub.2 gas is generated in the vicinity of the cathode-side copper electrode 14. Further, Cu(OH).sub.2 generated in the vicinity of the copper electrode 14 partially reacts as follows: EQU Cu(OH).sub.2 .fwdarw.CuO.dwnarw.+H.sub.2 O
CuO thus generated forms dendrite crystals called dendrites, which precipitate on the substrate surface 12 from the copper electrode 14 toward the copper electrode 13 to grow. When the CuO dendrites grow to cause a short across the copper electrodes 13 and 14, a malfunction takes place in the electronic circuit.
Such migration in a printed wiring board, generically named electromigration, occurs not only in copper electrodes but in other types of metal electrodes such as silver electrodes.
In order to prevent such electromigration on the surface of a printed wiring board, means of previously coating the surface with a resin film has already been carried out to attain an effect of preventing deterioration of surface insulation resistance.
However, the aforementioned copper migration occurring between through-holes is a phenomenon caused in clearances which are filled up with parts of copper through-hole plating layers. Therefore, it is impossible to prevent such copper migration by coating performed after copper plating, dissimilarly to the above case of migration occurring on the substrate surface.
Thus, a countermeasure for preventing some copper migration occurring in the process of manufacturing a printed wiring board has been awaited.
The phenomenon of migration caused between through-holes is not restricted to the copper migration in the case of copper through hole plating, but also occurs when through-holes are provided by coating of another type of conductive paint such as silver.